1. Field of the Invention:
This invention relates in general to lock pins for locking tubing hangers inside a wellhead of an oil or gas well, and in particular to a hydraulically actuated lock pin mechanism.
2. Description of the Prior Art:
A conventional means for securing a tubing hanger in a wellhead is by the use of a number of lock screws. The lock screws extend through radial passages in the wellhead, which are threaded. Each lock screw has a conical nose that engages a shoulder of the pipe hanger. Rotating the lock screw causes the nose to bear against the hanger, wedging it tightly in the wellhead against upward force.
The amount of downward force against the pipe hanger obtainable by conventional lock screws is limited. A considerable portion of the thrust load is lost due to friction between the rotating nose of the lock screw and the shoulder of the pipe hanger. As the screw is rotated against the hanger, a flat develops on the nose of the screw due to excessive bearing stress. A depression may also develop on the hanger. This increases the loss of load to friction as the nose begins to flatten. Greater torque results in further deformation of the nose, and eventually the nose of the screw will cease to rotate at all. This can result in torsional shear failure of the screw.
Traditionally, a sufficient number of lock screws were required to generate a total force capable of restraining a tubing hanger against the load from below. The load from below was previously based on the seal area of the tubing bowl less the area of the tubing string times the working pressure of the well. Recently, standards have been increased to require the tubing hanger to be locked down sufficiently to restrain a tubing hanger subjected to a full blind load of the rated working pressure of the tubing head top connection. In many cases, it is nearly impossible to install conventional lock screws of sufficient size, number and strength to meet this requirement. Too many locking screws will be detrimental to the working stress levels of the flange connector portion of the wellhead.